Quantum dot device and quantum dots

ABSTRACT

Quantum dots and electroluminescent devices including the same, wherein the quantum dots include a core including a first semiconductor nanocrystal including a zinc chalcogenide; and a shell disposed on the core, the shell including zinc, sulfur, and selenium, wherein the quantum dots have an average particle size of greater than 10 nm, wherein the quantum dots do not include cadmium, and wherein a photoluminescent peak of the quantum dots is present in a wavelength range of greater than or equal to about 430 nm and less than or equal to about 470 nm.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2018-0098842 filed in the Korean IntellectualProperty Office on Aug. 23, 2018, and all the benefits accruingtherefrom under 35 U.S.C. §119, the content of which in its entirety isherein incorporated by reference.

BACKGROUND 1. Field

A quantum dot device and quantum dots are disclosed.

2. Description of the Related Art

Physical characteristics (e.g., energy bandgaps, melting points, etc.)of nanoparticles that are intrinsic characteristics may be controlled bychanging the particle sizes of the nanoparticles, unlike bulk materials.For example, semiconductor nanocrystals also known as quantum dots maybe supplied with photoenergy or electrical energy and may emit light ina wavelength corresponding to sizes of the quantum dots. Accordingly,the quantum dots may be used as a light emitting element emitting lightof a particular wavelength.

SUMMARY

An embodiment provides a cadmium-free quantum dot-basedelectroluminescent device.

An embodiment provides the aforementioned quantum dots.

According to an embodiment, an electroluminescent light emitting deviceincludes

a first electrode and a second electrode facing each other, and

an emission layer disposed between the first electrode and the secondelectrode, the emission layer including quantum dots, wherein thequantum dots include a core including a first semiconductor nanocrystalincluding a zinc chalcogenide, and a shell disposed on the core, theshell including zinc, sulfur, and selenium,

wherein the quantum dots have an average particle size of greater than10 nanometers (nm), wherein the quantum dots do not include cadmium, anda photoluminescent peak of the quantum dots is present in a wavelengthrange of greater than or equal to about 430 nm and less than or equal toabout 470 nm.

The quantum dots may have an average particle size of greater than orequal to about 11 nm.

The quantum dots may have an average particle size of greater than orequal to about 12 nm.

The quantum dots may have an average particle size of greater than about12 nm.

A photoluminescent peak wavelength of the quantum dots may be greaterthan or equal to about 440 nm.

A photoluminescent peak wavelength of the quantum dots may be greaterthan or equal to about 450 nm.

The zinc chalcogenide may include zinc, selenium, and optionallytellurium.

The zinc chalcogenide may not include manganese, copper, or acombination thereof.

The zinc chalcogenide may include zinc, selenium, and tellurium and thequantum dots may have a mole ratio of tellurium with respect to seleniumof greater than or equal to about 0.001:1.

The zinc chalcogenide may include zinc, selenium, and tellurium and thequantum dots may have a mole ratio of tellurium with respect to seleniumof less than or equal to about 0.05:1.

The core may include ZnSe_(1-x)Te_(x), wherein, x is greater than orequal to about 0 and less than or equal to about 0.05.

The semiconductor nanocrystal shell has a gradient composition varyingin a radial direction from the core.

In the semiconductor nanocrystal shell, an amount (or a concentration)of the sulfur may increase toward a surface of the quantum dots.

The semiconductor nanocrystal shell may have a first layer directlydisposed on the core and at least one outer layer disposed on the firstlayer, wherein the first layer may include a second semiconductornanocrystal having a different composition from a composition of thefirst semiconductor nanocrystal and the outer layer may include a thirdsemiconductor nanocrystal having a different composition from acomposition of the second semiconductor nanocrystal.

The second semiconductor nanocrystal may include zinc, selenium, andoptionally tellurium, and the outer layer (or the third semiconductornanocrystal) may include zinc and sulfur.

The outer layer may be an outermost layer of the quantum dot.

The first semiconductor nanocrystal may include ZnSe_(1-x)Te_(x),wherein, x is greater than 0 and less than or equal to about 0.05, thesecond semiconductor nanocrystal may include a zinc selenide, and thethird semiconductor nanocrystal may include a zinc sulfide, and may notinclude selenium.

An energy bandgap of the first semiconductor nanocrystal may be lessthan an energy bandgap of the second semiconductor nanocrystal and theenergy bandgap of the second semiconductor nanocrystal may be less thanan energy bandgap of the third semiconductor nanocrystal.

An energy bandgap of the second semiconductor nanocrystal may be lessthan an energy bandgap of the first semiconductor nanocrystal and anenergy bandgap of the third semiconductor nanocrystal.

The quantum dots may have an average particle size of greater than orequal to about 15 nm.

The quantum dots may have an average value of solidity of greater thanor equal to about 0.85.

The quantum dots may have an average value of solidity of greater thanor equal to about 0.9.

The quantum dots may have a tetrahedron shape, a hexahedron shape (e.g.,cubic shape), an octahedron shape, or a combination thereof.

The quantum dots may have less than 4 pods on average.

An average number of the pods included in the quantum dots may be lessthan or equal to 3.

The quantum dot may include an organic ligand on a surface thereof, andthe organic ligand may include RCOOH, RNH₂, R₂NH, R₃N, RSH, RH2PO,R₂HPO, R₃PO, RH₂P, R₂HP, R₃P, ROH, RCOOR, RPO(OH)₂, RHPOOH, R₂POOH, or acombination thereof, wherein, each R is the same or different andindependently is a C1 to C40 substituted or unsubstituted aliphatichydrocarbon group, a C6 to C40 substituted or unsubstituted aromatichydrocarbon group, or a combination thereof.

The electroluminescent device may include a charge auxiliary layerbetween the first electrode and the quantum dot emission layer, betweenthe second electrode and the quantum dot emission layer, or between thefirst electrode and the quantum dot emission layer and between thesecond electrode and the quantum dot emission layer.

The electroluminescent device may have a maximum EQE of greater than orequal to about 3.0%, greater than or equal to about 3.6%, greater thanor equal to about 4.7%, greater than or equal to about 5%, greater thanor equal to about 5.5%, greater than or equal to about 6%, or greaterthan or equal to about 6.3%.

The electroluminescent device may have a maximum brightness of greaterthan or equal to about 1000 cd/m², greater than or equal to about 1360cd/m², greater than or equal to about 1950 cd/m², greater than or equalto about 2400 cd/m², greater than or equal to about 2500 cd/m², greaterthan or equal to about 2600 cd/m², greater than or equal to about 2700cd/m², or greater than or equal to about 2780 cd/m².

In an embodiment, a display device includes the aforementionedelectroluminescent device.

In an embodiment, the aforementioned quantum dots are provided.

In an embodiment, a composition including a liquid vehicle such as anorganic solvent and the aforementioned quantum dots is provided.

In an embodiment, quantum dots include a core; and a shell disposed onthe core, wherein a thickness of the shell may be greater than or equalto about 6 nanometers and less than or equal to about 50 nm, wherein thequantum dots have an average particle size of greater than 10nanometers, and wherein the quantum dots have an average value ofsolidity of greater than or equal to about 0.85:1.

The quantum dots of an embodiment may be environmentally friendly as thequantum dots do not include a toxic heavy metal such as lead, cadmium,mercury, or the like. The cadmium free quantum dots may be applied to,e.g., used in, various display devices and biolabeling (e.g., abiosensor or bioimaging), a photodetector, a solar cell, a hybridcomposite, and the like. The electroluminescent device of the embodimentincluding the aforementioned cadmium free quantum dots may show enhancedelectroluminescent properties.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages and features of this disclosure willbecome more apparent by describing in further detail exemplaryembodiments thereof with reference to the accompanying drawings, inwhich:

FIG. 1 is a view illustrating the concept of solidity of a particle.

FIG. 2 is a schematic cross-sectional view of a quantum dot lightemitting diode (QD LED) device according to an embodiment.

FIG. 3 is a schematic cross-sectional view of a QD LED device accordingto an embodiment.

FIG. 4 is a schematic cross-sectional view of a QD LED device accordingto an embodiment.

FIG. 5 is a transmission electron microscopic image of the quantum dotsprepared in Example 1.

FIG. 6 is a transmission electron microscopic image of the quantum dotsprepared in Example 2.

FIG. 7 is a transmission electron microscopic image of the quantum dotsprepared in Example 3.

FIG. 8 is a transmission electron microscopic image of the quantum dotsprepared in Comparative Example 1.

FIG. 9 is a transmission electron microscopic image of the quantum dotsprepared in Comparative Example 2.

FIG. 10 is a graph of external quantum efficiency (EQE) (percent (%))versus luminance (L) (candelas per square meter (Cd/m²)) showingelectro-luminescent properties of the devices according to Examples 4 to6 and Comparative Example 3.

DETAILED DESCRIPTION

Hereinafter, example embodiments of the present disclosure will bedescribed in detail so that a person skilled in the art would understandthe same. This disclosure may, however, be embodied in many differentforms and is not construed as limited to the example embodiments setforth herein.

In the drawings, the thickness of layers, films, panels, regions, etc.,are exaggerated for clarity. Like reference numerals designate likeelements throughout the specification. It will be understood that whenan element such as a layer, film, region, or substrate is referred to asbeing “on” another element, it can be directly on the other element orintervening elements may also be present. In contrast, when an elementis referred to as being “directly on” another element, there are nointervening elements present.

It will be understood that, although the terms “first,” “second,”“third” etc. may be used herein to describe various elements,components, regions, layers and/or sections, these elements, components,regions, layers and/or sections should not be limited by these terms.These terms are only used to distinguish one element, component, region,layer, or section from another element, component, region, layer, orsection. Thus, “a first element,” “component,” “region,” “layer,” or“section” discussed below could be termed a second element, component,region, layer, or section without departing from the teachings herein.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a,” “an,” and “the” are intended to include the pluralforms, including “at least one,” unless the content clearly indicatesotherwise. “At least one” is not to be construed as limiting “a” or“an.” “or” means “and/or.” As used herein, the term “and/or” includesany and all combinations of one or more of the associated listed items.It will be further understood that the terms “comprises” and/or“comprising,” or “includes” and/or “including” when used in thisspecification, specify the presence of stated features, regions,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,regions, integers, steps, operations, elements, components, and/orgroups thereof.

“About” as used herein is inclusive of the stated value and means withinan acceptable range of deviation for the particular value as determinedby one of ordinary skill in the art, considering the measurement inquestion and the error associated with measurement of the particularquantity (i.e., the limitations of the measurement system). For example,“about” can mean within one or more standard deviations, or within ±10%or 5% of the stated value.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

Exemplary embodiments are described herein with reference to crosssection illustrations that are schematic illustrations of idealizedembodiments. As such, variations from the shapes of the illustrations asa result, for example, of manufacturing techniques and/or tolerances,are to be expected. Thus, embodiments described herein should not beconstrued as limited to the particular shapes of regions as illustratedherein but are to include deviations in shapes that result, for example,from manufacturing. For example, a region illustrated or described asflat may, typically, have rough and/or nonlinear features. Moreover,sharp angles that are illustrated may be rounded. Thus, the regionsillustrated in the figures are schematic in nature and their shapes arenot intended to illustrate the precise shape of a region and are notintended to limit the scope of the present claims.

As used herein, a work function or an energy level (e.g., a highestoccupied molecular orbital (HOMO) energy level or a lowest unoccupiedmolecular orbital (LUMO) energy level) is expressed as an absolute valuefrom a vacuum level. In addition, when the work function or the energylevel is referred to be “deep,” “high” or “large,” the work function orthe energy level has a large absolute value based on “0 eV” of thevacuum level, while when the work function or the energy level isreferred to be “shallow,” “low,” or “small,” the work function or energylevel has a small absolute value based on “0 eV” of the vacuum level.

As used herein, the term “Group” may refer to a group of Periodic Table.

As used herein, “Group I” may refer to Group IA and Group IB, andexamples may include Li, Na, K, Rb, and Cs, but are not limited thereto.

As used herein, “Group II” may refer to Group IIA and Group IIB, andexamples of Group II metal may be Cd, Zn, Hg, and Mg, but are notlimited thereto.

As used herein, “Group III” may refer to Group IIIA and Group IIIB, andexamples of Group III metal may be Al, In, Ga, and TI, but are notlimited thereto.

As used herein, “Group IV” may refer to Group IVA and Group IVB, andexamples of a Group IV metal may be Si, Ge, and Sn, but are not limitedthereto. As used herein, the term “metal” may include a semi-metal suchas Si. As used herein, “Group V” may refer to Group VA, and examples mayinclude nitrogen, phosphorus, arsenic, antimony, and bismuth, but arenot limited thereto.

As used herein, “Group VI” may refer to Group VIA, and examples mayinclude sulfur, selenium, and tellurium, but are not limited thereto.

As used herein, when a definition is not otherwise provided,“substituted” refers to replacement of hydrogen of a compound, a group,or a moiety by a C1 to C30 alkyl group, a C2 to C30 alkenyl group, a C2to C30 alkynyl group, a C2 to C30 epoxy group, a C2 to C30 alkyl estergroup, a C3 to C30 alkenyl ester group (e.g., an acrylate group,methacrylate group), a C6 to C30 aryl group, a C7 to C30 alkylarylgroup, a 01 to C30 alkoxy group, a 01 to C30 heteroalkyl group, a C3 toC40 heteroaryl group, a C3 to C30 heteroarylalkyl group, a C3 to C30heteroalkylaryl group, a C3 to C30 cycloalkyl group, a C3 to C15cycloalkenyl group, a C6 to C30 cycloalkynyl group, a C2 to C30heterocycloalkyl group, a halogen (—F, —Cl, —Br, or —I), a hydroxy group(—OH), a nitro group (—NO₂), a thiocyanate group (—SCN), a cyano group(—CN), an amino group (—NRR′ wherein R and R′ are independently hydrogenor a C1 to C6 alkyl group), an azido group (—N₃), an amidino group(—C(═NH)NH₂), a hydrazino group (—NHNH₂), a hydrazono group (═N(NH₂)),an aldehyde group (—C(═O)H), a carbamoyl group (—C(O)NH₂), a thiol group(—SH), an ester group (—C(═O)OR, wherein R is a C1 to C6 alkyl group ora C6 to C12 aryl group), a carboxyl group (—COOH) or a salt thereof(—C(═O)OM, wherein M is an organic or inorganic cation), a sulfonic acidgroup (—SO₃H) or a salt thereof (—SO₃M, wherein M is an organic orinorganic cation), a phosphoric acid group (—PO₃H₂) or a salt thereof(—PO₃MH or —PO₃M₂, wherein M is an organic or inorganic cation), or acombination thereof.

As used herein, when a definition is not otherwise provided, the term“hydrocarbon group” refers to a group including carbon and hydrogen(e.g., alkyl, alkenyl, alkynyl, aryl, or the like). The hydrocarbongroup may be a group having a valence of at least one formed by aremoval of at least one hydrogen from an alkane, an alkene, an alkyne,an arene, or the like. At least one methylene in the hydrocarbon groupmay be replaced with an oxide moiety, a carbonyl moiety, an estermoiety, —NH—, or a combination thereof.

As used herein, when a definition is not otherwise provided, “aliphatic”refers to a C1 to C40 linear or branched hydrocarbon (e.g., alkyl,alkenyl, or alkynyl) group.

As used herein, when a definition is not otherwise provided, “alkoxy”refers to an alkyl group that is linked via an oxygen (i.e., alkyl-O—),for example, methoxy, ethoxy, and sec-butyloxy groups.

As used herein, when a definition is not otherwise provided, “alkyl”refers to a straight or branched chain, saturated, monovalent (e.g., C1to C40) hydrocarbon group (e.g., methyl or hexyl).

As used herein, when a definition is not otherwise provided, “alkylene”refers to a straight or branched saturated (e.g., C2 to C40) aliphatichydrocarbon group having a valence of at least two and optionallysubstituted with at least one substituent.

As used herein, when a definition is not otherwise provided, “alkynyl”refers to a straight or branched chain, monovalent (e.g., C2 to C40)hydrocarbon group having at least one carbon-carbon triple bond (e.g.,ethynyl).

As used herein, when a definition is not otherwise provided, an “amine”group has the general formula —NRR, wherein each R is independentlyhydrogen, a C1-012 alkyl group, a C7-C20 alkylaryl group, a C7-C20arylalkyl group, or a C6-C18 aryl group.

As used herein, when a definition is not otherwise provided, “arene”refers to a hydrocarbon having an aromatic ring, and includes monocyclicand polycyclic hydrocarbons wherein the additional ring(s) of thepolycyclic hydrocarbon may be aromatic or nonaromatic. Specific examplesof arenes include benzene, naphthalene, toluene, and xylene.

As used herein, when a definition is not otherwise provided, “aromatic”refers to an organic compound or group comprising at least oneunsaturated cyclic group having delocalized pi electrons. The termencompasses both hydrocarbon aromatic compounds and heteroaromaticcompounds.

As used herein, when a definition is not otherwise provided, “aryl”refers to a hydrocarbon group having a valence of at least one, forexample, formed by the removal of at least one hydrogen atom from one ormore rings of an arene (e.g., phenyl or naphthyl).

As used herein, when a definition is not otherwise provided, “arylalkyl”refers to a substituted or unsubstituted aryl group covalently linked toan alkyl group that is linked to a compound (e.g., a benzyl is a C7arylalkyl group).

As used herein, when a definition is not otherwise provided, “arylene”refers to a functional group having a valence of at least two obtainedby removal of at least two hydrogens in at least one aromatic ring, andoptionally substituted with at least one substituent.

As used herein, when a definition is not otherwise provided, the term“hetero” refers to inclusion of at least one (e.g., one to three)heteroatoms, where the heteroatom(s) may be N, O, S, Si, or P,preferably N, O, or S.

As used herein, when a definition is not otherwise provided,“heteroalkyl” refers to an alkyl group that comprises at least oneheteroatom covalently bonded to one or more carbon atoms of the alkylgroup.

As used herein, when a definition is not otherwise provided,“heteroaryl” refers to an aromatic group that comprises at least oneheteroatom covalently bonded to one or more carbon atoms of aromaticring.

The wording “average” used in this specification (e.g., an averageparticle size, an average size of the quantum dot, an average value ofsolidity, an average number of the pods) may be mean or median. In anembodiment, the average may be “mean” average.

Hereinafter, a light emitting device according to an embodiment isdescribed with reference to drawings.

FIG. 2 is a schematic cross-sectional view of an electroluminescentdevice (hereinafter, also referred to as a light emitting device)according to an embodiment.

Referring to FIG. 2, a light emitting device 10 according to anembodiment includes a first electrode 11 and a second electrode 15facing each other, and an emission layer 13 disposed between the firstelectrode 11 and the second electrode 15 and including quantum dots. Ahole auxiliary layer 12 may be disposed between the first electrode 11and the emission layer 13, and an electron auxiliary layer 14 may bedisposed between the second electrode 15 and the emission layer 13.

The device may further include a substrate. The substrate may bedisposed on a major (e.g., lower) surface of the first electrode 11 oron a major (e.g., upper) surface of the second electrode 15. In anembodiment, the substrate may be disposed on a major (e.g., lower)surface of the first electrode. The substrate may be a substrateincluding an insulation material (e.g., insulating transparentsubstrate). The substrate may include glass; a polymer such as apolyester (e.g., polyethylene terephthalate (PET), polyethylenenaphthalate (PEN)), a polycarbonate, a polyacrylate, a polyimide, apolyamideimide, or a combination thereof; a polysiloxane (e.g., PDMS);an inorganic material such as Al₂O₃, ZnO, or a combination thereof; or acombination thereof, but is not limited thereto. The substrate may bemade of a silicon wafer. Herein “transparent” may refer to transmittancefor light of a predetermined wavelength (e.g., light emitted from thequantum dots) of greater than or equal to about 85%, for example,greater than or equal to about 88%, greater than or equal to about 90%,greater than or equal to about 95%, greater than or equal to about 97%,or greater than or equal to about 99%. A thickness of the substrate maybe appropriately selected taking into consideration a substrate materialbut is not particularly limited. The transparent substrate may haveflexibility. The substrate may be omitted.

One of the first electrode 11 and the second electrode 15 may be ananode and the other may be a cathode. For example, the first electrode11 may be an anode and the second electrode 15 may be a cathode.

The first electrode 11 may be made of a conductor, for example, a metal,a conductive metal oxide, or a combination thereof. The first electrode11 may be, for example, made of a metal or an alloy thereof such asnickel, platinum, vanadium, chromium, copper, zinc, or gold; aconductive metal oxide such as zinc oxide, indium oxide, tin oxide,indium tin oxide (ITO), indium zinc oxide (IZO), or fluorine doped tinoxide; or a combination of a metal and a metal oxide such as ZnO and Alor SnO₂ and Sb, but is not limited thereto. In an embodiment, the firstelectrode may include a transparent conductive metal oxide, for example,indium tin oxide. A work function of the first electrode may be higherthan a work function of the second electrode that will be describedlater. A work function of the first electrode may be lower than a workfunction of the second electrode.

The second electrode 15 may be made of a conductor, for example, ametal, a conductive metal oxide, a conductive polymer, or a combinationthereof. The second electrode 15 may include, for example, a metal or analloy thereof such as aluminum, magnesium, calcium, sodium, potassium,titanium, indium, yttrium, lithium, gadolinium, silver, tin, lead,cesium, or barium; a multi-layer structured material such as LiF/Al,Li₂O/Al, Liq/Al, LiF/Ca, or BaF₂/Ca, but is not limited thereto. Theconductive metal oxide is the same as described above.

In an embodiment, a work function of the first electrode 11 may be fromabout 4.5 electronvolts (eV) to about 5.0 eV (e.g., from about 4.6 eV toabout 4.9 eV). The work function of the second electrode 15 may begreater than or equal to about 4.0 eV and less than about 4.5 eV (e.g.,from about 4.0 eV to about 4.3 eV). In an embodiment, a work function ofthe second electrode 15 may be from about 4.5 eV to about 5.0 eV (e.g.,from about 4.6 eV to about 4.9 eV). The work function of the firstelectrode 11 may be greater than or equal to about 4.0 eV and less thanabout 4.5 eV (e.g., from about 4.0 eV to about 4.3 eV). A work functionof the first electrode may be lower than a work function of the secondelectrode. A work function of the first electrode may be higher than awork function of the second electrode.

The first electrode 11, the second electrode 15, or a combinationthereof may be a light-transmitting electrode and the light-transmittingelectrode may be, for example, made of a conductive oxide such as a zincoxide, indium oxide, tin oxide, indium tin oxide (ITO), indium zincoxide (IZO), or fluorine doped tin oxide, or a metal thin layer of asingle layer or a multilayer. When one of the first electrode 11 and thesecond electrode 15 is a non-light-transmitting electrode, thenon-light-transmitting electrode may be made of, for example, an opaqueconductor such as aluminum (Al), silver (Ag), or gold (Au).

A thickness of the electrodes (the first electrode, the secondelectrode, or a combination thereof) is not particularly limited and maybe appropriately selected taking into consideration device efficiency.For example, the thickness of the electrodes may be greater than orequal to about 5 nm, for example, greater than or equal to about 50 nm.For example, the thickness of the electrodes may be less than or equalto about 100 micrometers (μm), for example, less than or equal to about10 μm, less than or equal to about 1 μm, less than or equal to about 900nm, less than or equal to about 500 nm, or less than or equal to about100 nm.

The emission layer 13 includes (e.g., a plurality of) quantum dots. Thequantum dots (hereinafter, also referred to as semiconductornanocrystals) may absorb light from an excitation source to be excitedand may emit energy corresponding to an energy bandgap of the quantumdot. The energy bandgap of the quantum dot may vary with a size and acomposition of the semiconductor nanocrystal. For example, as the sizeof a quantum dot increases, the quantum dot may have a narrower energybandgap, thereby having an increased luminous wavelength. Semiconductornanocrystals may be used as a light emitting material in various fieldssuch as a display device, an energy device, or a bio light emittingdevice.

Quantum dots having a photoluminescence (PL) property at an applicablelevel may include cadmium (Cd). Cadmium may cause environment/healthproblems and is one of the restricted elements via Restriction ofHazardous Substances Directive (RoHS) in many countries. Accordingly,development of a cadmium-free semiconductor nanocrystal particle havingimproved photoluminescence characteristics is desired. However,developing cadmium free quantum dots emitting blue light and exhibitingdesired electroluminescent properties (e.g., a narrower full width athalf maximum (FWHM), enhanced EQE, increased brightness, or the like)and an electroluminescent device including the same may be difficult. Inan embodiment, in order to realize a QD-LED device having increasedbrightness and enhanced color gamut, a blue light emitting quantum dothaving a PL peak of about 450 nm to about 460 nm, an increased PLquantum yield (QY), and a narrow FWHM may be provided. To date, noreport known to the inventors has been made for cadmium free quantum dotthat allows the aforementioned electroluminescent properties in adevice. Luminous properties of a ZnSe/ZnS core/shell structure may bedifficult to control at a wavelength of greater than or equal to about440 nm. An increase of particle size beyond a predetermined level maycause an adverse effect on the efficiency of the quantum dot. Thepresent inventors have been found that a thick shell may be advantageousfor securing stability and efficiency required for the application to adevice. However, the increase of the shell thickness may also result inan irregular morphology of the quantum dot and may tend to result in adecrease of a photoluminescent efficiency.

In an embodiment, a blue light emitting quantum dot may be prepared bycontrolling a shell growth condition (e.g., a reaction temperature, aninjection rate of precursor(s), or other factors affecting a shellgrowth rate and types and amounts of precursor(s)) to have a compositionthat will be described below, and whereby the quantum dots of theembodiment may have both of an increased particle size and a desiredmorphology (for example, a solidity that will be described later). Thequantum dots of the embodiments may exhibit relatively enhanced externalquantum efficiency (EQE) and increased brightness. Thus, an embodimentis related to the quantum dots (e.g., a population of the quantum dots).

In an embodiment, the quantum dots (e.g., included in the light emittinglayer of the electroluminescent device) may include a core including afirst semiconductor nanocrystal including a zinc chalcogenide; and ashell disposed on the core and including zinc, sulfur, and selenium. Thequantum dots have an average particle size of greater than 10 nm, forexample, greater than or equal to about 11 nm, greater than or equal toabout 12 nm, or greater than about 12 nm. The quantum dot does notinclude cadmium. The quantum dots are configured to emit blue light. Anaverage value of solidity (hereinafter, also referred to as solidity) ofthe quantum dots may be greater than or equal to about 0.85, forexample, greater than or equal to about 0.88, greater than or equal toabout 0.9, greater than or equal to about 0.91, or greater than or equalto about 0.92.

As used herein, the term “solidity” refers to a ratio of an area (B) ofa two dimensional area of a quantum dot with respect to an area (A) of aconvex hull. The convex hull may be defined as the smallest convex setof points in which a set of all points constituting a two dimensionalimage of a given quantum dot obtained by an electron microscopicanalysis is contained. Stated otherwise, the convex hull may be definedas a convex polygon of the smallest area in which a set of all pointsconstituting a two dimensional image of a given quantum dot obtained byan electron microscopic analysis is contained. (see FIG. 1) The soliditymay be measured by a transmission electron microscopic analysis. Forexample, a commercially-available computer program (e.g., an imageprocessing program such as “image J”) may be used to calculate (anaverage value of) solidity from a transmission electron microscopy (TEM)image of the quantum dots.

The electroluminescent device of an embodiment includes theaforementioned quantum dots and thereby may exhibit a desired level ofelectroluminescent properties, even when they are based on a cadmiumfree quantum dot.

The first semiconductor nanocrystal included in the core of the quantumdots may include a zinc chalcogenide including zinc, selenium, andoptionally tellurium. The zinc chalcogenide may not include manganese,copper, or a combination thereof. The first semiconductor nanocrystalmay include a limited amount of tellurium. The core may includeZnSe_(1-x)Te_(x) (wherein, x is greater than or equal to about 0 andless than or equal to about 0.05).

A size (e.g., a diameter) of the core may be greater than or equal toabout 1.5 nm, greater than or equal to about 2 nm, greater than or equalto about 2.1 nm, greater than or equal to about 2.2 nm, greater than orequal to about 2.3 nm, greater than or equal to about 2.4 nm, or greaterthan or equal to about 2.5 nm. A size of the core may be less than orequal to about 5 nm, less than or equal to about 4.9 nm, less than orequal to about 4.8 nm, less than or equal to about 4.7 nm, less than orequal to about 4.6 nm, less than or equal to about 4.5 nm, less than orequal to about 4.4 nm, less than or equal to about 4.3 nm, less than orequal to about 4.2nm, less than or equal to about 4.1 nm, less than orequal to about 4 nm, less than or equal to about 3.9 nm, less than orequal to about 3.8 nm, less than or equal to about 3.7 nm, less than orequal to about 3.6 nm, or less than or equal to about 3.5 nm. In anembodiment, a size may refer to an average size. The average may be“mean” average.

The quantum dots may have less than 4 pods on average.

An average number of the pods included in the quantum dots may be lessthan or equal to 3.

The semiconductor nanocrystal shell has a gradient composition varyingin a radial direction from the core. In the semiconductor nanocrystalshell, an amount (e.g., a concentration) of the sulfur may increasetoward a surface of the quantum dots.

The semiconductor nanocrystal shell may include at least two layers andadjacent layers may have a different composition. In an embodiment, thesemiconductor nanocrystal shell may include a first layer disposeddirectly on the core and at least one outer layer (e.g., a second layer,a third layer, etc.) disposed on the first layer. The first layer mayinclude a second semiconductor nanocrystal and the outer layer (e.g.,the second layer or the third layer) may include a third semiconductornanocrystal having a different composition from the second semiconductornanocrystal.

The second semiconductor nanocrystal may include zinc, selenium, andoptionally tellurium. In an embodiment, the second semiconductornanocrystal may include ZnSe, ZnSeS, or a combination thereof. The outerlayer (or the third semiconductor nanocrystal) may include zinc andsulfur, and optionally a selenium. In an embodiment, the outer layer (orthe third semiconductor nanocrystal) may include ZnSeS, ZnS, or acombination thereof.

An outermost layer of the quantum dot may include a semiconductornanocrystal consisting of zinc and sulfur.

In the quantum dots included in the electroluminescent device of theembodiment, the first semiconductor nanocrystal may includeZnSe_(1-x)Te_(x) (wherein, x is greater than 0 and less than or equal toabout 0.05), the second semiconductor nanocrystal may include a ZnSe,and the third semiconductor nanocrystal may include a ZnS and may notinclude selenium.

In an embodiment, an energy bandgap of the first semiconductornanocrystal may be less than that of the second semiconductornanocrystal and an energy bandgap of the second semiconductornanocrystal may be less than that of the third semiconductor nanocrystal(Type 1). In an embodiment, an energy bandgap of the secondsemiconductor nanocrystal may be less than those of the firstsemiconductor nanocrystal and the third semiconductor nanocrystal(Reverse Type 1).

A thickness of a shell may be selected taking into consideration acomposition and a size of the core. In an embodiment, the thickness ofthe shell may be greater than or equal to about 4 nm, greater than orequal to about 5 nm, greater than or equal to about 6 nm, greater thanor equal to about 6.5 nm, greater than or equal to about 7 nm, greaterthan or equal to about 7.5 nm, greater than or equal to about 8 nm,greater than or equal to about 8.5 nm, greater than or equal to about 9nm, greater than or equal to about 9.5 nm, greater than or equal toabout 10 nm, greater than or equal to about 10.5 nm, greater than orequal to about 11 nm, greater than or equal to about 12 nm, or greaterthan or equal to about 13 nm. The thickness of the shell may be lessthan or equal to about 50 nm, less than or equal to about 40 nm, lessthan or equal to about 30 nm, less than or equal to about 20 nm, lessthan or equal to about 19 nm, less than or equal to about 18 nm, or lessthan or equal to about 17 nm. A thickness of the shell may be in a rangeof 4 nm to 10 nm

In the quantum dots, if present, a molar amount of tellurium withrespect to one mole of selenium may be greater than or equal to about0.0001 moles, greater than or equal to about 0.0005 moles, greater thanor equal to about 0.0006 moles, greater than or equal to about 0.0007moles, greater than or equal to about 0.0008 moles, greater than orequal to about 0.0009 moles, greater than or equal to about 0.001 moles,greater than or equal to about 0.002 moles, greater than or equal toabout 0.003 moles, greater than or equal to about 0.004 moles, greaterthan or equal to about 0.005 moles, greater than or equal to about 0.006moles, greater than or equal to about 0.007 moles, greater than or equalto about 0.008 moles, greater than or equal to about 0.009 moles,greater than or equal to about 0.01 moles, greater than or equal toabout 0.02 moles, or greater than or equal to about 0.025 moles. In thecore or the quantum dot, an amount of the tellurium with respect to onemole of selenium may be less than or equal to about 0.053 moles, lessthan or equal to about 0.05 moles, less than or equal to about 0.049moles, less than or equal to about 0.048 moles, less than or equal toabout 0.047 moles, less than or equal to about 0.046 moles, less than orequal to about 0.045 moles, less than or equal to about 0.044 moles,less than or equal to about 0.043 moles, less than or equal to about0.042 moles, less than or equal to about 0.041 moles, less than or equalto about 0.04 moles, less than or equal to about 0.035 moles, less thanor equal to about 0.03, less than or equal to about 0.02 moles, lessthan or equal to about 0.0135 moles, less than or equal to about 0.013moles, less than or equal to about 0.01 moles, less than or equal toabout 0.009 moles, less than or equal to about 0.008 moles, less than orequal to about 0.007 moles, or less than or equal to about 0.006 moles.

In the quantum dots or the cores thereof, an amount of tellurium withrespect to one mole of zinc may be greater than or equal to about 0.0001moles, greater than or equal to about 0.0005 moles, greater than orequal to about 0.0006 moles, greater than or equal to about 0.0007moles, greater than or equal to about 0.0008 moles, greater than orequal to about 0.0009 moles, greater than or equal to about 0.001 moles,greater than or equal to about 0.002 moles, greater than or equal toabout 0.003 moles, greater than or equal to about 0.004 moles, greaterthan or equal to about 0.005 moles, greater than or equal to about 0.006moles, greater than or equal to about 0.007 moles, greater than or equalto about 0.008 moles, greater than or equal to about 0.009 moles,greater than or equal to about 0.01 moles, greater than or equal toabout 0.02 moles, or greater than or equal to about 0.025 moles.

In the quantum dots or the cores thereof, an amount of tellurium withrespect to one mole of zinc may be less than or equal to about 0.053moles, less than or equal to about 0.05 moles, less than or equal toabout 0.049 moles, less than or equal to about 0.048 moles, less than orequal to about 0.047 moles, less than or equal to about 0.046 moles,less than or equal to about 0.045 moles, less than or equal to about0.044 moles, less than or equal to about 0.043 moles, less than or equalto about 0.042 moles, less than or equal to about 0.041 moles, less thanor equal to about 0.04 moles, less than or equal to about 0.035 moles,less than or equal to about 0.03 moles, 0.02 moles, less than or equalto about 0.01 moles, less than or equal to about 0.009 moles, less thanor equal to about 0.008 moles, less than or equal to about 0.007 moles,less than or equal to about 0.006 moles, less than or equal to about0.005 moles, less than or equal to about 0.004 moles, or less than orequal to about 0.003 moles.

In the quantum dot according to an embodiment, a molar ratio of the sumof sulfur and selenium with respect to zinc ((Se+S)/Zn) may be greaterthan or equal to about 0.5:1, greater than or equal to about 0.6:1,greater than or equal to about 0.7:1, greater than or equal to about0.8:1, or greater than or equal to about 0.85:1. In the quantum dotaccording to an embodiment, a molar ratio of the sum of sulfur andselenium with respect to zinc ((Se+S)/Zn) may be less than or equal toabout 1.1:1, less than or equal to about 1.05:1, less than or equal toabout 1.0:1, less than or equal to about 0.99:1, less than or equal toabout 0.98:1, less than or equal to about 0.97:1, less than or equal toabout 0.96:1, or less than or equal to about 0.95:1.

In the quantum dot according to an embodiment, a molar ratio of sulfurwith respect to selenium may be greater than or equal to about 0.4,greater than or equal to about 0.5:1, greater than or equal to about0.6:1, greater than or equal to about 0.7:1, greater than or equal toabout 0.75:1, greater than or equal to about 0.76:1, greater than orequal to about 0.77:1, greater than or equal to about 0.78:1, greaterthan or equal to about 0.79:1, or greater than or equal to about 0.8:1.In the quantum dot according to an embodiment, a molar ratio of sulfurwith respect to selenium may be less than or equal to about 2:1, lessthan or equal to about 1.9:1, less than or equal to about 1.8:1, lessthan or equal to about 1.7:1, less than or equal to about 1.6:1, lessthan or equal to about 1.55, less than or equal to about 1.5, less thanor equal to about 1.45, less than or equal to about 1.4, or less than orequal to about 1.37.

The quantum dots included in the device of the embodiment may have arelatively large particle size. In an embodiment, an average size of thequantum dot may be greater than or equal to about 10 nm. In anembodiment, an average size of the aforementioned quantum dots may begreater than 10 nm, greater than or equal to about 10.5 nm, greater thanor equal to about 11 nm, greater than or equal to about 11.5 nm, greaterthan or equal to about 12 nm, greater than or equal to about 12.3 nm,greater than or equal to about 12.4 nm, greater than or equal to about12.5 nm, greater than or equal to about 12.6 nm, greater than or equalto about 12.7 nm, greater than or equal to about 12.8 nm, greater thanor equal to about 12.9 nm, greater than or equal to about 13 nm, greaterthan or equal to about 13.5 nm, greater than or equal to about 14 nm,greater than or equal to about 14.5 nm, greater than or equal to about15 nm, greater than or equal to about 15.5 nm, or greater than or equalto about 16 nm. An average size of the aforementioned quantum dots maybe less than or equal to about 50 nm, less than or equal to about 45 nm,less than or equal to about 40 nm, less than or equal to about 35 nm,less than or equal to about 30 nm, less than or equal to about 29 nm,less than or equal to about 28 nm, less than or equal to about 27 nm,less than or equal to about 26 nm, less than or equal to about 25 nm,less than or equal to about 24 nm, less than or equal to about 23 nm,less than or equal to about 22 nm, less than or equal to about 21 nm, orless than or equal to about 20 nm. In the embodiment, a size of thequantum dot may be a diameter (or an equivalent diameter) calculatedfrom a two dimensional image obtained from an (transmission) electronmicroscope analysis under an assumption that the image is a roundcircle.

The quantum dots included in the device of the embodiment have a shapeof a relatively increased density even when the quantum dots haverelatively large sizes. A convex area of an object may be defined as anarea of a convex hull enclosing the object and a convex hull of anobject may be defined as the smallest convex shape that contains theobject. Thus, the solidity may represent a shape density of a givenobject and the measurement of the solidity may be obtained as a ratio ofthe area of the object with respect to the area of the convex hull ofthe object.

In an embodiment, the solidity of the quantum dots of the embodiment maybe greater than or equal to about 0.86, greater than or equal to about0.87, greater than or equal to about 0.88, greater than or equal toabout 0.89, greater than or equal to about 0.90, greater than or equalto about 0.91, or greater than or equal to about 0.92. The quantum dotsof the embodiment may have the solidity of the aforementioned rangestogether with the aforementioned sizes and the quantum dots may exhibitimproved electroluminescent properties.

A photoluminescent peak wavelength of the quantum dots may be greaterthan or equal to about 430 nm, greater than or equal to about 440 nm,greater than or equal to about 445 nm, or greater than or equal to about450 nm and less than or equal to about 480 nm, less than or equal toabout 475 nm, less than or equal to about 470 nm, or less than or equalto about 465 nm.

The quantum dots of the embodiment may emit blue light that has amaximum peak wavelength of about 450 nm to about 460 nm.

A maximum luminescent peak wavelength of the electroluminescent deviceof an embodiment (Lamda Max) may be greater than or equal to about 440nm, greater than or equal to about 445 nm, greater than or equal toabout 450 nm, greater than or equal to about 451 nm, or greater than orequal to about 452 nm and less than or equal to about 470 nm, less thanor equal to about 465 nm, or less than or equal to about 460 nm.

The maximum emission peak of the quantum dots may have a FWHM of lessthan or equal to about 40 nm, less than or equal to about 39 nm, lessthan or equal to about 38 nm, less than or equal to about 37 nm, lessthan or equal to about 36 nm, less than or equal to about 35 nm, lessthan or equal to about 34 nm, less than or equal to about 33 nm, lessthan or equal to about 32 nm, less than or equal to about 31 nm, lessthan or equal to about 30 nm, less than or equal to about 29 nm, lessthan or equal to about 28 nm, or less than or equal to about 27 nm.

The quantum dots may have a quantum efficiency (hereinafter, alsoreferred to as a quantum yield) of greater than or equal to about 50%,for example, greater than or equal to about 60%, greater than or equalto about 61%, greater than or equal to about 62%, greater than or equalto about 63%, greater than or equal to about 64%, greater than or equalto about 65%, greater than or equal to about 66%, greater than or equalto about 67%, greater than or equal to about 68%, greater than or equalto about 69%, greater than or equal to about 70%, greater than or equalto about 71%, greater than or equal to about 72%, greater than or equalto about 73%, greater than or equal to about 74%, greater than or equalto about 75%, greater than or equal to about 76%, greater than or equalto about 77%, greater than or equal to about 78%, greater than or equalto about 79%, or greater than or equal to about 80%.

The quantum dots may have an organic ligand on a surface thereof. Theorganic ligand may include, RCOOH, RNH₂, R₂NH, R₃N, RSH, RH₂PO, R₂HPO,R₃PO, RH₂P, R₂HP, R₃P, ROH, RCOOR′, RPO(OH)₂, R₂POOH, or a combinationthereof, wherein, R and R′ independently are a C1 to C40 substituted orunsubstituted aliphatic hydrocarbon group, a C6 to C40 substituted orunsubstituted aromatic hydrocarbon group, or a combination thereof.

In an embodiment, the quantum dots included in the device of theembodiment may be prepared by a method, which includes:

obtaining a core including a first semiconductor nanocrystal includingzinc, selenium, and tellurium (hereinafter referred to as a “core”);reacting a zinc precursor in an organic solvent in the presence of thecore including the first semiconductor nanocrystal and the organicligand with a non-metal precursor of a selenium precursor, a sulfurprecursor, or a combination thereof, a plurality of times, to form asemiconductor nanocrystal shell including zinc and selenium, and sulfuron a surface of the core at a relatively high temperature (e.g., ofgreater than or equal to about 300° C., or at least 320° C. such asgreater than or equal to about 330° C. or higher).

The temperature for a shell formation may be less than or equal to about360° C., or less than or equal to about 350° C.

The non-metal precursors may be intermittently injected in a splitmanner (e.g., at a predetermined amount divided from a desired totalamount) at least two times taking into consideration a desired thicknessand composition of the shell.

The formation of the semiconductor nanocrystal shell may includereacting the zinc precursor with the selenium precursor and optionally asulfur precursor and then reacting the zinc precursor with the sulfurprecursor.

In an embodiment, the core may be obtained by preparing a zinc precursorsolution including a zinc precursor and an organic ligand; preparing aselenium precursor and a tellurium precursor; and heating the zincprecursor solution up to a first reaction temperature and adding theselenium precursor and the tellurium precursor and an organic ligand andperforming a reaction.

The zinc precursor may be a Zn metal powder, ZnO, an alkylated Zncompound (e.g., C2 to C30 dialkyl zinc such as diethyl zinc), a Znalkoxide (e.g., zinc ethoxide), a Zn carboxylate (e.g., zinc acetate), aZn nitrate, a Zn perchlorate, a Zn sulfate, Zn acetylacetonate, a Znhalide (e.g., zinc chloride), a Zn cyanide, a Zn hydroxide, or acombination thereof. Examples of the zinc precursor may be dimethylzinc, diethyl zinc, zinc acetate, zinc acetylacetonate, zinc iodide,zinc bromide, zinc chloride, zinc fluoride, zinc carbonate, zinccyanide, zinc nitrate, zinc oxide, zinc peroxide, zinc perchlorate, zincsulfate, or a combination thereof.

The selenium precursor may include selenium-trioctylphosphine (Se-TOP),selenium-tributylphosphine (Se-TBP), selenium-triphenylphosphine(Se-TPP), selenium-diphenylphosphine (Se-DPP), or a combination thereof,but is not limited thereto.

The tellurium precursor may include tellurium-tributylphosphine(Te-TBP), tellurium-triphenylphosphine (Te-TPP),tellurium-diphenylphosphine (Te-DPP), or a combination thereof, but isnot limited thereto.

The sulfur precursor may be hexane thiol, octane thiol, decane thiol,dodecane thiol, hexadecane thiol, mercapto propyl silane,sulfur-trioctylphosphine (S-TOP), sulfur-tributylphosphine (S-TBP),sulfur-triphenylphosphine (S-TPP), sulfur-trioctylamine (S-TOA),bistrimethylsilyl sulfur, ammonium sulfide, sodium sulfide, or acombination thereof.

The organic solvent may include a C6 to C22 primary amine such ashexadecylamine, a C6 to C22 secondary amine such as dioctylamine, a C6to C40 tertiary amine such as trioctylamine, a nitrogen-containingheterocyclic compound such as pyridine, a C6 to C40 olefin such asoctadecene, a C6 to C40 aliphatic hydrocarbon such as hexadecane,octadecane, or squalane, an aromatic hydrocarbon substituted with a C6to C30 alkyl group such as phenyldodecane, phenyltetradecane, or phenylhexadecane, a primary, secondary, or tertiary phosphine substituted withat least one (e.g., 1, 2, or 3) C6 to C22 alkyl group (e.g.,trioctylphosphine), a phosphine oxide substituted with at least one(e.g., 1, 2, or 3) C6 to C22 alkyl group (e.g., trioctyl phosphineoxide), a C12 to C22 aromatic ether such as phenyl ether or benzylether, or a combination thereof.

The organic ligand may coordinate, e.g., bind to, the surface of theproduced nanocrystal and may have an effect on light emitting andelectric characteristics and may effectively disperse the nanocrystal inan organic solvent. The organic ligand may include RCOOH, RNH₂, R₂NH,R₃N, RSH, RH₂PO, R₂HPO, R₃PO, RH₂P, R₂HP, R₃P, ROH, RCOOR′, RPO(OH)₂,R₂POOH, or a combination thereof, wherein, R and R′ independently are aC1 to C24 (C3 to C40) substituted or unsubstituted aliphatic hydrocarbongroup, a C6 to C20 (C6-C40) substituted or unsubstituted aromatichydrocarbon group, or a combination thereof. One or more organic ligandsmay be used.

Specific examples of the organic ligand compound may be methane thiol,ethane thiol, propane thiol, butane thiol, pentane thiol, hexane thiol,octane thiol, dodecane thiol, hexadecane thiol, octadecane thiol, benzylthiol; methane amine, ethane amine, propane amine, butane amine, pentaneamine, hexane amine, octane amine, dodecane amine, hexadecyl amine,oleyl amine, octadecyl amine, dimethyl amine, diethyl amine, dipropylamine; methanoic acid, ethanoic acid, propanoic acid, butanoic acid,pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, dodecanoicacid, hexadecanoic acid, octadecanoic acid, oleic acid, benzoic acid,palmitic acid, or stearic acid; a phosphine such as methyl phosphine,ethyl phosphine, propyl phosphine, butyl phosphine, pentyl phosphine,tributyl phosphine, or trioctyl phosphine; a phosphine oxide compoundsuch as methyl phosphine oxide, ethyl phosphine oxide, propyl phosphineoxide, butyl phosphine oxide, or trioctyl phosphine oxide; a diphenylphosphine or triphenyl phosphine compound, or an oxide compound thereof;a C5 to C20 alkyl phosphonic acid such as hexylphosphinic acid,octylphosphinic acid, dodecanephosphinic acid, tetradecanephosphinicacid, hexadecanephosphinic acid, octadecanephosphinic acid; phosphonicacid; and the like, but are not limited thereto. One or more organicligand compounds may be used. In an embodiment, the organic ligandcompound may be a combination of RCOOH and an amine (e.g., RNH₂, R₂NH,R₃N, or a combination thereof) wherein, R is independently a C1 to C24(C3-C40) substituted or unsubstituted aliphatic hydrocarbon group or aC6 to C20 (C6-C40) substituted or unsubstituted aromatic hydrocarbongroup. In an embodiment R is independently a C1 to C24 (C3-C40)substituted or unsubstituted aliphatic hydrocarbon group.

In the core including the first semiconductor nanocrystal, a mole ratioof tellurium with respect to selenium may be less than or equal to about0.05:1. In order to form the core, an amount of the selenium precursorduring formation of the core may be greater than or equal to about 20moles, for example, greater than or equal to about 25 moles, greaterthan or equal to about 26 moles, greater than or equal to about 27moles, greater than or equal to about 28 moles, greater than or equal toabout 29 moles, greater than or equal to about 30 moles, greater than orequal to about 31 moles, greater than or equal to about 32 moles,greater than or equal to about 33 moles, greater than or equal to about34 moles, greater than or equal to about 35 moles, greater than or equalto about 36 moles, greater than or equal to about 37 moles, greater thanor equal to about 38 moles, greater than or equal to about 39 moles, orgreater than or equal to about 40 moles, with respect to 1 mole of thetellurium precursor during formation of the core. The amount of theselenium precursor may be less than or equal to about 60 moles, lessthan or equal to about 59 moles, less than or equal to about 58 moles,less than or equal to about 57 moles, less than or equal to about 56moles, or less than or equal to about 55 moles, with respect to 1 moleof the tellurium precursor. Within the amount ranges, the core havingthe composition described above may be formed.

The first reaction temperature may be greater than or equal to about280° C., for example, greater than or equal to about 290° C. A reactiontime for forming the core is not particularly limited and may beappropriately selected. For example, the reaction time may be greaterthan or equal to about 5 minutes, greater than or equal to about 10minutes, greater than or equal to about 15 minutes, greater than orequal to about 20 minutes, greater than or equal to about 25 minutes,greater than or equal to about 30 minutes, greater than or equal toabout 35 minutes, greater than or equal to about 40 minutes, greaterthan or equal to about 45 minutes, or greater than or equal to about 50minutes, but is not limited thereto. For example, the reaction time maybe less than or equal to about 2 hours but is not limited thereto. Bycontrolling the reaction time, the size of the core may be adjusted.

In a method of an embodiment, a reaction condition for a shell growth iscontrolled and thereby a shell composition and a growth rate thereof arecontrolled, which makes it possible to provide a quantum dot having anincreased particle size and the aforementioned solidity at the sametime.

In a method of an embodiment, a solvent and optionally an organic ligandis heated (or vacuum treated) at a predetermined temperature (e.g.,greater than or equal to about 100° C.) in a flask, then an atmospherein the flask is converted into an inert gas atmosphere, and the solventand optionally the organic ligand are heated again at a predeterminedtemperature (e.g., greater than or equal to about 100° C.).Subsequently, the core is added, and the shell precursors are added stepby step (e.g., in multiple steps) at a predetermined reactiontemperature to conduct a reaction. The number of the steps and theinjected amounts of the precursors may be determined taking intoconsideration a reactivity of each of the shell precursors and a desiredcomposition of the shell (e.g., having a gradient concentration or amulti-layered structure). In an embodiment, a selenium precursor (andoptionally a sulfur precursor) may be injected in a plurality of thesteps (e.g., intermittently) to form a first layer. Then, apredetermined amount of a sulfur precursor is added over a plurality ofsteps (e.g., intermittently) to form an outer layer. A reactiontemperature for a shell formation may be greater than or equal to about300° C., greater than or equal to about 310° C., greater than or equalto about 320° C., greater than or equal to about 325° C., or greaterthan or equal to about 330° C. The temperature for a shell formation maybe less than or equal to about 360° C., or less than or equal to about350° C.

After completing the reaction, a nonsolvent is added to reactionproducts and nanocrystal particles coordinated with, e.g., bound to, theligand compound may be separated. The nonsolvent may be a polar solventthat is miscible with the solvent used in the core formation reaction,shell formation reaction, or a combination thereof and is not capable ofdispersing the produced nanocrystals therein. The nonsolvent may beselected depending the solvent used in the reaction and may be, forexample, acetone, ethanol, butanol, isopropanol, ethanediol, water,tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), diethyl ether,formaldehyde, acetaldehyde, ethylene glycol, a solvent having a similarsolubility parameter to the foregoing solvents, or a combinationthereof. The nanocrystal particles may be separated throughcentrifugation, sedimentation, chromatography, or distillation. Theseparated nanocrystals may be added to a washing solvent and washed, ifdesired. The washing solvent has no particular limit and may have asimilar solubility parameter to that of the ligand and may include, forexample, hexane, heptane, octane, chloroform, toluene, benzene, and thelike.

The quantum dots of the embodiment may not be dispersible in water, anyof the foregoing listed non-solvents, or a mixture thereof. The quantumdots of the embodiment may be water-insoluble.

The quantum dots of the embodiments may be dispersed in theaforementioned organic solvent. In some embodiments, the quantum dotsmay be dispersed in a C6 to C40 aliphatic hydrocarbon, a C6 to C40aromatic hydrocarbon, or a mixture thereof.

In an embodiment, the emission layer 13 may include a monolayer ofquantum dots. In an embodiment, the emission layer 13 may include atleast one monolayer of quantum dots, for example, at least 2 monolayers,at least 3 monolayers, or at least 4 monolayers, and less than or equalto about 20 monolayers, less than or equal to about 10 monolayers, lessthan or equal to about 9 monolayers, less than or equal to about 8monolayers, less than or equal to about 7 monolayers, or less than orequal to about 6 monolayers. The emission layer 13 may have a thicknessof greater than or equal to about 5 nm, for example, greater than orequal to about 10 nm, greater than or equal to about 20 nm, or greaterthan or equal to about 30 nm and less than or equal to about 200 nm, forexample, less than or equal to about 150 nm, less than or equal to about100 nm, less than or equal to about 90 nm, less than or equal to about80 nm, less than or equal to about 70 nm, less than or equal to about 60nm, or less than or equal to about 50 nm. The emission layer 13 mayhave, for example, a thickness of about 10 nm to about 150 nm, forexample, about 10 nm to about 100 nm, for example, about 10 nm to about50 nm.

The device of an embodiment may further include a hole auxiliary layer.In a non-limiting embodiment, the hole auxiliary layer 12 is disposedbetween the first electrode 11 and the emission layer 13. The holeauxiliary layer 12 may have one layer or two or more layers, and mayinclude, for example, a hole injection layer (HIL), a hole transportlayer (HTL), an electron blocking layer, or a combination thereof.

The hole auxiliary layer 12 may have a HOMO energy level that may matcha HOMO energy level of the emission layer 13 and may enforce, e.g., aid,mobility of holes from the hole auxiliary layer 12 into the emissionlayer 13.

The HOMO energy level of the hole auxiliary layer 12 (e.g., holetransport layer (HTL)) contacting the emission layer may be the same asor less than the HOMO energy level of the emission layer 13 within arange of less than or equal to about 1.0 eV. For example, a differenceof HOMO energy levels between the hole auxiliary layer 12 and theemission layer 13 may be about 0 eV to about 1.0 eV, for example, about0.01 eV to about 0.9 eV, about 0.01 eV to about 0.8 eV, about 0.01 eV toabout 0.7 eV, about 0.01 eV to about 0.5 eV, about 0.01 eV to about 0.4eV, about 0.01 eV to about 0.3 eV, about 0.01 eV to about 0.2 eV, orabout 0.01 eV to about 0.1 eV.

In an embodiment, the hole auxiliary layer 12 may include a holeinjection layer nearer to the first electrode 11 and a hole transportlayer nearer to the emission layer 13. Herein, the HOMO energy level ofthe hole injection layer may be about 5.0 eV to about 5.3 eV and theHOMO energy level of the hole transport layer may be about 5.2 eV toabout 5.5 eV.

A material included in the hole auxiliary layer 12 (for example, a holetransporting layer or a hole injection layer) is not particularlylimited and may include, for example,poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB),polyarylamine, poly(N-vinylcarbazole), poly(3,4-ethylenedioxythiophene)(PEDOT), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate(PEDOT:PSS), polyaniline, polypyrrole,N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine (TPD),4-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (a-NPD), m-MTDATA(4,4′,4″-tris[phenyl(m-tolyl)amino]triphenylamine),4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA),1,1-bis[(di-4-tolylamino)phenylcyclohexane (TAPC), a p-type metal oxide(e.g., NiO, WO₃, MoO₃, etc.), a carbon-based material such as grapheneoxide, or a combination thereof, but is not limited thereto.

The electron blocking layer (EBL) may include, for example,poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS),poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB)polyarylamine, poly(N-vinylcarbazole), polyaniline, polypyrrole,N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine (TPD),4-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (a-NPD), m-MTDATA,4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA), or a combinationthereof, but is not limited thereto.

In the hole auxiliary layer(s), a thickness of each layer may beappropriately selected depending on the desired characteristics of eachlayer. For example, the thickness of each layer may be greater than orequal to about 10 nm, for example, greater than or equal to about 15 nm,greater than or equal to about 20 nm and less than or equal to about 100nm, for example, less than or equal to about 90 nm, less than or equalto about 80 nm, less than or equal to about 70 nm, less than or equal toabout 60 nm, less than or equal to about 50 nm, less than or equal toabout 40 nm, less than or equal to about 35 nm, or less than or equal toabout 30 nm, but is not limited thereto.

The electron auxiliary layer 14 is disposed between the emission layer13 and the second electrode 15. The electron auxiliary layer 14 mayinclude, for example, an electron injection layer (EIL) facilitating theinjection of the electrons, an electron transport layer (ETL)facilitating the transport of the electrons, a hole blocking layer (HBL)blocking the hole movement, or a combination thereof, but is not limitedthereto.

In an embodiment, the EIL may be disposed between the ETL and thecathode. In an embodiment, the HBL may be disposed between the ETL (orthe EIL) and the emissive layer, but is not limited thereto. In anembodiment, a thickness of each layer may be greater than or equal toabout 1 nm and less than or equal to about 500 nm, but is not limitedthereto. The EIL may be an organic layer (e.g., prepared by vapordeposition). The ETL may include an inorganic oxide nanoparticle, anorganic layer (e.g., prepared by vapor deposition), or a combinationthereof.

The electron transport layer, the electron injection layer, or acombination thereof may include, for example,1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA), bathocuproine(BCP), tris[3-(3-pyridyl)-mesityl]borane (3TPYMB), LiF, Alq₃, Gaq₃,Inq₃, Znq₂, Zn(BTZ)₂, BeBq₂, ET204(8-(4-(4,6-di(naphthalen-2-yl)-1,3,5-triazin-2-yl)phenyl)quinolone),8-hydroxyquinolinato lithium (Liq), an n-type metal oxide (e.g., ZnO,HfO₂, etc.), or a combination thereof, but is not limited thereto.

The hole blocking layer (HBL) may include, for example,1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA), bathocuproine(BCP), tris[3-(3-pyridyl)-mesityl]borane (3TPYMB), LiF, Alq₃, Gaq₃,Inq₃, Znq₂, Zn(BTZ)₂, BeBq₂, or a combination thereof, but is notlimited thereto.

In an embodiment, the electron auxiliary layer 14 may include anelectron transport layer. The ETL may include a plurality ofnanoparticles.

The nanoparticles include a metal oxide including zinc.

The metal oxide may include zinc oxide, zinc magnesium oxide, or acombination thereof. The metal oxide may include Zn_(1-x)M_(x)O (whereinM is Mg, Ca, Zr, W, Li, Ti, or a combination thereof and 0≤x≤0.5). In anembodiment, the M may be magnesium (Mg). In an embodiment, x may begreater than or equal to about 0.01 and less than or equal to about 0.3,for example, less than or equal to about 0.25, less than or equal toabout 0.2, or less than or equal to about 0.15.

An absolute value of LUMO of quantum dots included in the emission layermay be smaller than an absolute value of LUMO of the metal oxide. In anembodiment, an absolute value of LUMO of quantum dots may be larger thanan absolute value of LUMO of a metal oxide ETL. An average size of thenanoparticles may be greater than or equal to about 1 nm, for example,greater than or equal to about 1.5 nm, greater than or equal to about 2nm, greater than or equal to about 2.5 nm, or greater than or equal toabout 3 nm and less than or equal to about 10 nm, less than or equal toabout 9 nm, less than or equal to about 8 nm, less than or equal toabout 7 nm, less than or equal to about 6 nm, or less than or equal toabout 5 nm. The nanoparticles may not have a rod shape. Thenanoparticles may not have a nano wire shape.

In an embodiment, the thickness of the electron auxiliary layer 14(e.g., the thickness of each of an electron injection layer, an electrontransport layer, or a hole blocking layer) may be greater than or equalto about 5 nm, greater than or equal to about 6 nm, greater than orequal to about 7 nm, greater than or equal to about 8 nm, greater thanor equal to about 9 nm, greater than or equal to about 10 nm, greaterthan or equal to about 11 nm, greater than or equal to about 12 nm,greater than or equal to about 13 nm, greater than or equal to about 14nm, greater than or equal to about 15 nm, greater than or equal to about16 nm, greater than or equal to about 17 nm, greater than or equal toabout 18 nm, greater than or equal to about 19 nm, or greater than orequal to about 20 nm and less than or equal to about 120 nm, less thanor equal to about 110 nm, less than or equal to about 100 nm, less thanor equal to about 90 nm, less than or equal to about 80 nm, less than orequal to about 70 nm, less than or equal to about 60 nm, less than orequal to about 50 nm, less than or equal to about 40 nm, less than orequal to about 30 nm, or less than or equal to about 25 nm, but is notlimited thereto.

In a device according to an embodiment, an anode 10 disposed on atransparent substrate 100 may include a metal oxide-based transparentelectrode (e.g., ITO electrode) and a cathode 50 facing the anode mayinclude a metal (Mg, Al, etc.) of a relatively low work function. Forexample, a hole auxiliary layer 20, for example, a hole transport layerincluding TFB, poly(9-vinylcarbazole) (PVK), or a combination thereof; ahole injection layer including PEDOT:PSS, a p-type metal oxide, or acombination thereof; or a combination thereof may be disposed betweenthe transparent electrode 10 and the emission layer 30. An electronauxiliary layer 40 such as an electron injection layer/transport layermay be disposed between the quantum dot emission layer 30 and thecathode 50. (see FIG. 3)

A device according to an embodiment has an inverted structure. Herein,the cathode 50 disposed on a transparent substrate 100 may include ametal oxide-based transparent electrode (e.g., ITO) and the anode 10facing the cathode may include a metal (e.g., Au, Ag, etc.) of arelatively high work function. For example, an n-type metal oxide (ZnO)may be disposed between the transparent electrode 50 and the emissionlayer 30 as an electron auxiliary layer 40 (e.g., an electron transportlayer (ETL)). A hole auxiliary layer 20 (e.g., a hole transport layer(HTL) including TFB, PVK, or a combination thereof a hole injectionlayer (HIL) including MoO₃ or another p-type metal oxide, or acombination thereof) may be disposed between the metal anode 10 and thequantum dot emission layer 30 as a hole auxiliary layer (e.g., holetransport layer (HTL)). (see FIG. 4)

The device of the embodiment may be prepared in an appropriate manner.In an embodiment, the electroluminescent device may be prepared byforming a charge (e.g., hole) auxiliary layer on a substrate having anelectrode thereon (e.g., via deposition or coating), forming an emissivelayer including the quantum dots (e.g., a pattern of the aforementionedquantum dots) thereon (e.g., via deposition or coating), and forming anelectrode (optionally together with a charge (e.g., electron) auxiliarylayer) thereon (e.g., via deposition or coating). The formation of theelectrode/hole auxiliary layer/electron auxiliary layer is notparticularly limited and may be selected appropriately.

Formation of the emissive layer may be carried out by preparing an inkcomposition including the aforementioned quantum dots of the embodimentand a liquid vehicle, and depositing the prepared ink composition (forexample, via an ink jet printing method or the like). Accordingly, anembodiment is related to an ink composition including the aforementionedquantum dots and a liquid vehicle. The ink composition may furtherinclude a light diffusing particle, a binder (e.g., a binder having acarboxylic acid group), and optionally at least one additive (e.g., amonomer having a carbon-carbon double bond, a crosslinker, an initiator,or the like). The light diffusing particle may include TiO₂, SiO₂,BaTiO₃, ZnO, or a combination thereof. The light diffusing particle mayhave a size of greater than or equal to about 100 nm and less than orequal to about 1 μm.

The liquid vehicle may include an organic solvent. The liquid vehicle(e.g., the organic solvent) are not particularly limited. Types andamounts of the organic solvent may be appropriately selected by takinginto consideration the aforementioned main components (i.e., the quantumdot, the COOH group-containing binder, the photopolymerizable monomer,the photoinitiator, and if used, the thiol compound), and types andamounts of additives which will be described below. Non-limitingexamples of the liquid vehicle may include, but are not limited to:ethyl 3-ethoxy propionate; an ethylene glycol series such as ethyleneglycol, diethylene glycol, or polyethylene glycol; a glycol ether suchas ethylene glycol monomethyl ether, ethylene glycol monoethyl ether,diethylene glycol monomethyl ether, ethylene glycol diethyl ether, ordiethylene glycol dimethyl ether; a glycol ether acetate such asethylene glycol monomethyl ether acetate, ethylene glycol monoethylether acetate, diethylene glycol monoethyl ether acetate, or diethyleneglycol monobutyl ether acetate; a propylene glycol series such aspropylene glycol; a propylene glycol ether such as propylene glycolmonomethyl ether, propylene glycol monoethyl ether, propylene glycolmonopropyl ether, propylene glycol monobutyl ether, propylene glycoldimethyl ether, dipropylene glycol dimethyl ether, propylene glycoldiethyl ether, or dipropylene glycol diethyl ether; a propylene glycolether acetate such as propylene glycol monomethyl ether acetate ordipropylene glycol monoethyl ether acetate; an amide such asN-methylpyrrolidone, dimethyl formamide, or dimethyl acetamide; a ketonesuch as methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), orcyclohexanone; a petroleum product such as toluene, xylene, or solventnaphtha; an ester such as ethyl acetate, propyl acetate, butyl acetate,cyclohexyl acetate, or ethyl lactate; an ether such as diethyl ether,dipropyl ether, or dibutyl ether; a C1 to C40 aliphatic hydrocarbon(e.g., an alkane, alkene, or alkyne); a halogen- (e.g.,chloro-)substituted C1 to C40 aliphatic hydrocarbon (e.g.,dichloroethane, trichloromethane, or the like); a C6 to C40 aromatichydrocarbon (e.g., toluene, xylene, or the like); a halogen (e.g.,chloro) substituted C6 to C40 aromatic hydrocarbon; or a combinationthereof.

Components included in the ink composition and concentrations thereofmay be adjusted to control a viscosity of the ink composition, which isnot particularly limited. A viscosity of the ink composition may be lessthan or equal to about 20 centipoise (cP), less than or equal to about15 cP, less than or equal to about 10 cP, less than or equal to about 5cP, less than or equal to about 4 cP, less than or equal to about 3 cP,less than or equal to about 2 cP, or less than or equal to about 1.5 cP.The viscosity of the ink composition may be greater than or equal toabout 0.1 cP, greater than or equal to about 0.5 cP, or greater than orequal to about 0.8 cP.

In an embodiment, an electronic device includes the aforementionedquantum dots. The device may include a display device, a light emittingdiode (LED), an organic light emitting diode (OLED), a quantum dot LED,a sensor, a solar cell, an image sensor, or a liquid crystal display(LCD), but is not limited thereto. In an embodiment, the electronicdevice may include a photoluminescent device (e.g., a quantum dot sheetor a lighting such as a quantum dot rail or a liquid crystal display(LCD)). In a non-limiting embodiment, the electronic device may includea quantum dot sheet and the aforementioned quantum dots are dispersed inthe sheet (e.g., in the form of a quantum dot polymer composite).

Hereinafter, the embodiments are illustrated in more detail withreference to examples. However, these examples are exemplary, and thepresent scope is not limited thereto.

EXAMPLES Analysis Method 1. Photoluminescence Analysis

Photoluminescence (PL) spectra of the prepared quantum dots are obtainedusing a Hitachi F-7000 spectrometer at an irradiation wavelength of 372nanometers (nm).

2. Ultraviolet-Visible (UV-Vis) Spectroscopic Analysis

Hitachi U-3310 spectrometer is used to perform a UV spectroscopicanalysis and obtain UV-Visible absorption spectra.

3. Transmission Electron Microscopy (TEM) Analysis

Transmission electron microscopy photographs of quantum dots areobtained using an UT F30 Tecnai electron microscope.

4. ICP Analysis

An inductively coupled plasma-atomic emission spectroscopy (ICP-AES)analysis is performed using Shimadzu ICPS-8100.

5. Electroluminescence Spectroscopic Analysis

A current depending on a voltage is measured using a Keithley 2635Bsource meter while applying a voltage and electroluminescent (EL) lightemitting luminance is measured using a CS2000 spectrometer.

Synthesis is performed under an inert gas atmosphere (nitrogen flowingcondition) unless particularly mentioned.

Synthesis of Quantum Dots Example 1: (QD3)

(1) Selenium and tellurium are dispersed in trioctylphosphine (TOP) toobtain a Se/TOP stock solution and a Te/TOP stock solution,respectively.

4.5 millimoles (mmol) of zinc acetate along with oleic acid is dissolvedin trioctylamine in a 250 milliliter (mL) reaction flask and then,heated at 120° C. under vacuum. After 1 hour, the atmosphere in thereactor is converted to an inert gas.

After heating the solution at 300° C., the Se/TOP stock solution and theTe/TOP stock solution produced above are rapidly injected thereinto at aratio of Te:Se=1:30. The amount of Se with respect to one mole of Zn is0.5 moles. After the completion of the reaction, the obtained reactionsolution is cooled down to room temperature, ethanol is added thereto,the obtained mixture is centrifuged to obtain a precipitate, and theprecipitate is dispersed in toluene to obtain a toluene dispersion ofZnSeTe core. An average size of the core is about 3 nm.

(2) Trioctylamine (TOA) is put in a 250 mL reaction flask, zinc acetateand oleic acid are added thereto, and the obtained mixture is treatedunder vacuum at 120° C. The atmosphere inside of the flask is replacedwith nitrogen (N₂). The flask is heated up to 330° C. or 340° C. and thetoluene dispersion of the ZnSeTe core is rapidly added thereto. Apredetermined amount of the Se/TOP stock solution is intermittentlyadded thereto three times while the reaction proceeds for 90 minutes toform a ZnSe layer on the core. Then, to the reaction flask, apredetermined amount of zinc acetate and a predetermined amount of aS/TOP stock solution are intermittently added thereto three times,respectively while another reaction proceeds for 30 minutes to form aZnS layer. Herein, the used amounts of a Zn precursor, a Se precursor,and a S precursor are 3.0 mmol, 0.8 mmol, and 2.8 mmol, respectively.

(3) Characterization

Photoluminescence characteristics of a quantum dot produced therefromare analyzed, and the results are shown in Table 1.

An inductively coupled plasma atomic emission spectroscopic (ICP-AES)analysis of the produced quantum dot is made, and the results are shownin Table 2.

A transmission electron microscope analysis is made for the quantum dotsthus prepared and the results are shown in FIG. 5. The results of FIG. 5confirm that the prepared quantum dots have an average size of about12.3 nm and a solidity of about 0.90:1. An average thickness of theshell is estimated to be about 4.65 (9.3/2) nm.

Example 2: QD4

(1) Core-shell quantum dots (QD4) are prepared in the same manner as setforth in Example 1 except that the used amounts of the zinc precursor,the Se precursor, and the S precursor are 3.6 mmol, 1.2 mmol, and 2.8mmol, respectively.

(2) Characterization

Photoluminescence characteristics of a quantum dot produced therefromare analyzed, and the results are shown in Table 1.

An inductively coupled plasma atomic emission spectroscopic (ICP-AES)analysis of the produced quantum dot is made, and the results are shownin Table 2.

A transmission electron microscope analysis is made for the quantum dotsthus prepared and the results are shown in FIG. 6. The results of FIG. 6confirm that the prepared quantum dots have an average size of about16.1 nm and a solidity of about 0.92:1. An average thickness of theshell is estimated to be about 6.55 nm (13.1/2) nm.

Example 3: QD5

(1) Core-shell quantum dots (QD5) are prepared in the same manner as setforth in Example 1 except that the used amounts of the zinc precursor,the Se precursor, and the S precursor are 4.5 mmol, 1.2 mmol, and 2.8mmol, respectively.

(2) Characterization

Photoluminescence characteristics of a quantum dot produced therefromare analyzed, and the results are shown in Table 1.

An inductively coupled plasma atomic emission spectroscopic (ICP-AES)analysis of the produced quantum dot is made, and the results are shownin Table 2.

A transmission electron microscope analysis is made for the quantum dotsthus prepared and the results are shown in FIG. 7. The results of FIG. 7confirm that the prepared quantum dots have an average size of about19.5 nm and a solidity of about 0.89:1. An average thickness of theshell is estimated to be about 8.25 (16.5/2) nm.

Comparative Example 1: QD1

(1) Core-shell quantum dots (QD1) are prepared in the same manner as setforth in Example 1 except that the used amounts of the zinc precursor,the Se precursor, and the S precursor are 3.0 mmol, 0.8 mmol, and 2.8mmol, respectively, the reaction temperature is 320° C., and the Seprecursor and the S precursor are added in a single step, respectively.

(2) Characterization

Photoluminescence characteristics of a quantum dot produced therefromare analyzed, and the results are shown in Table 1.

An inductively coupled plasma atomic emission spectroscopic (ICP-AES)analysis of the produced quantum dot is made, and the results are shownin Table 2.

A transmission electron microscope analysis is made for the quantum dotsthus prepared and the results are shown in FIG. 8. The results of FIG. 8confirm that the prepared quantum dots have an average size of about 9.1nm and a solidity of about 0.94:1. An average thickness of the shell isestimated to be about 3.05 (6.1/2) nm.

Comparative Example 2: QD2

(1) Core-shell quantum dots (QD2) are prepared in the same manner as setforth in Example 1 except that the used amounts of the zinc precursor,the Se precursor, and the S precursor are 2 mmol, 0.5 mmol, and 1.4mmol, respectively, the reaction temperature is 320° C., and the Seprecursor and the S precursor are added in a single step, respectively.

(2) Characterization

Photoluminescence characteristics of a quantum dot produced therefromare analyzed, and the results are shown in Table 1.

An inductively coupled plasma atomic emission spectroscopic (ICP-AES)analysis of the produced quantum dot is made, and the results are shownin Table 2.

A transmission electron microscope analysis is made for the quantum dotsthus prepared and the results are shown in FIG. 9. The results of FIG. 9confirm that the prepared quantum dots have an average size of about11.6 nm and a solidity of about 0.83:1. An average thickness of theshell is estimated to be about 4.3 (8.6/2) nm.

TABLE 1 Full width PL quantum at half Average yield maximum Particle(QY) (%) (FWHM) (nm) Solidity size (nm) Comp. Example 1 82 18 0.94 9.1(QD1) Comp. Example 2 85 22 0.83 11.6 (QD2) Example 1 (QD3) 86 24 0.9012.3 Example 2 (QD4) 72 24 0.92 16.1 Example 3 (QD5) 63 22 0.89 19.5

TABLE 2 ICP -AES Results S:Zn Zn:Zn Se:Zn Te:Zn (Se + S):Zn Comp.Example 1 0.22:1 1.00:1 0.65:1 0.002:1 0.87:1 (QD1) Comp. Example 20.48:1 1.00:1 0.43:1 — 0.91:1 (QD2) Example 1 (QD3) 0.52:1 1.00:1 0.38:10.002:1 0.90:1 Example 2 (QD4) 0.40:1 1.00:1 0.51:1 0.002:1 0.91:1Example 3 (QD5) 0.53:1 1.00:1 0.34:1 0.002:1 0.87:1

The results of Tables 1 and 2 confirm that the quantum dots of theExamples have compositions, sizes, and shapes different from those ofthe quantum dots prepared in Comparative Examples.

Production of Electroluminescent Device

Example 4

An electroluminescent device is manufactured by using the quantum dot(QD3) of Example 1:

A poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)layer and apoly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB)layer are spin-coated to respectively form a hole injection layer HILand a hole transport layer (HTL) on a glass substrate deposited with anindium tin oxide (ITO) electrode (an anode).

On the TFB layer, a quantum dot emission layer is formed by spin-coatingan octane dispersion of the quantum dot. On the quantum dot emissionlayer, a film based on crystalline zinc oxide nanoparticles (thickness:about 40 nm) is formed as the electron auxiliary layer, and then, an Alelectrode is deposited thereon.

Electroluminescent properties of the device are measured and the resultsare shown in FIG. 10 and Table 3.

Example 5

An electroluminescent device is manufactured in the same manner as setforth in Example 4, except for using the quantum dot (QD4) of Example 2.

Electroluminescent properties of the device are measured and the resultsare shown in FIG. 10 and Table 3.

Example 6

An electroluminescent device is manufactured in the same manner as setforth in Example 4, except for using the quantum dot (QD5) of Example 3.

Electroluminescent properties of the device are measured and the resultsare shown in FIG. 10 and Table 3.

Comparative Example 3

An electroluminescent device is manufactured in the same manner as setforth in Example 4, except for using the quantum dot (QD1) of Comp.Example 1.

Electroluminescent properties of the device are measured and the resultsare shown in FIG. 10 and Table 3.

Comparative Example 4

An electroluminescent device is manufactured in the same manner as setforth in Example 4, except for using the quantum dot (QD2) of Comp.Example 2.

Electroluminescent properties of the device are measured and the resultsare summarized in Table 3.

TABLE 3 Maximum Maximum EQE @ Photolumi- External 100 nits nescenceQuantum (candelas Maximum Wavelength Efficiency per square luminance(Lambda (EQE) (%) meter) (%) (Cd/m²) maximum) Comp. Ex 3 (QD1) 2.6 2.5830 453 Comp. Ex 4 (QD2) 2.4 1.9 520 453 Example 4 (QD3) 3.6 3.6 1360452 Example 5 (QD4) 4.7 4.5 1950 452 Example 6(QD5) 6.3 6.1 2780 452

The results of FIG. 10 and Table 3 confirm that the electroluminescentdevice including the quantum dots of the Examples may exhibit improvedelectroluminescent properties in comparison with those of the deviceincluding the quantum dots of Comparative Examples.

While this disclosure has been described in connection with what ispresently considered to be practical example embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A electroluminescent device, comprising a firstelectrode and a second electrode facing each other, an emission layerdisposed between the first electrode and the second electrode, theemission layer comprising quantum dots, and wherein the quantum dotscomprise a core comprising a first semiconductor nanocrystal comprisinga zinc chalcogenide, and a shell disposed on the core, the shellcomprising zinc, sulfur, and selenium, wherein the quantum dots have anaverage particle size of greater than 10 nanometers, wherein the quantumdots do not comprise cadmium, and wherein a photoluminescent peak of thequantum dots is present in a wavelength range of greater than or equalto about 430 nanometers and less than or equal to about 470 nanometers.2. The electroluminescent device of claim 1, wherein the zincchalcogenide comprises zinc, selenium, and optionally tellurium.
 3. Theelectroluminescent device of claim 1, wherein the core does not comprisemanganese, copper, or a combination thereof.
 4. The electroluminescentdevice of claim 1, wherein the zinc chalcogenide comprises zinc,selenium, and tellurium, and wherein the quantum dots have a mole ratioof tellurium with respect to selenium of greater than or equal to about0.001:1 and less than or equal to about 0.05:1.
 5. Theelectroluminescent device of claim 1, wherein the core comprisesZnSe_(1-x)Te_(x), wherein, x is greater than or equal to 0 and less thanor equal to about 0.05.
 6. The electroluminescent device of claim 1,wherein the semiconductor nanocrystal shell comprises a concentrationgradient in a radial direction.
 7. The electroluminescent device ofclaim 1, wherein in the semiconductor nanocrystal shell, an amount ofthe sulfur increases toward a surface of the quantum dots.
 8. Theelectroluminescent device of claim 1, wherein the semiconductornanocrystal shell comprises a first layer disposed directly on the core,and an outer layer disposed on the first layer, and wherein the firstlayer comprises a second semiconductor nanocrystal having a compositiondifferent from a composition of the first semiconductor nanocrystal andwherein the outer layer comprises a third semiconductor nanocrystalhaving a composition different from the composition of the secondsemiconductor nanocrystal.
 9. The electroluminescent device of claim 8,wherein the second semiconductor nanocrystal comprises zinc, selenium,and optionally tellurium, and the outer layer comprises zinc and sulfur.10. The electroluminescent device of claim 8, wherein the outer layer isan outermost layer of the quantum dot.
 11. The electroluminescent deviceof claim 8, wherein the first semiconductor nanocrystal comprisesZnSe_(1-x)Te_(x), wherein, x is greater than 0 and less than or equal toabout 0.05, the second semiconductor nanocrystal comprises a ZnSe, andthe third semiconductor nanocrystal comprises a ZnS and does notcomprises selenium.
 12. The electroluminescent device of claim 8,wherein an energy bandgap of the first semiconductor nanocrystal is lessthan an energy bandgap of the second semiconductor nanocrystal and theenergy bandgap of the second semiconductor nanocrystal is less than anenergy bandgap of the third semiconductor nanocrystal.
 13. Theelectroluminescent device of claim 8, wherein an energy bandgap of thesecond semiconductor nanocrystal is less than an energy bandgap of thefirst semiconductor nanocrystal and an energy bandgap of the thirdsemiconductor nanocrystal.
 14. The electroluminescent device of claim 1,wherein the quantum dots have an average particle size of greater thanabout 12 nanometers.
 15. The electroluminescent device of claim 1,wherein the quantum dots have an average value of solidity of greaterthan or equal to about 0.85.
 16. The electroluminescent device of claim1, wherein the quantum dots have an average value of solidity of greaterthan or equal to about 0.9.
 17. The electroluminescent device of claim1, wherein the quantum dots have a tetrahedron shape, a hexahedronshape, an octahedron shape, or a combination thereof.
 18. Theelectroluminescent device of claim 1, the quantum dots comprise anorganic ligand on a surface thereof, and the organic ligand comprisesRCOOH, RNH₂, R₂NH, R₃N, RSH, RH₂PO, R₂HPO, R₃PO, RH₂P, R₂HP, R₃P, ROH,RCOOR, RPO(OH)₂, RHPOOH, RHPOOH, or a combination thereof, wherein, R isthe same or different and independently is a C1 to C40 substituted orunsubstituted aliphatic hydrocarbon group, a C6 to C40 substituted orunsubstituted aromatic hydrocarbon group, or a combination thereof. 19.The electroluminescent device of claim 1, wherein the electroluminescentdevice comprises a charge auxiliary layer between the first electrodeand the quantum dot emission layer, between the second electrode and thequantum dot emission layer, or between the first electrode and thequantum dot emission layer and between the second electrode and thequantum dot emission layer.
 20. Quantum dots comprising a corecomprising a first semiconductor nanocrystal comprising a zincchalcogenide; and a shell disposed on the core, the shell comprisingzinc, sulfur, and selenium, wherein the quantum dots have an averageparticle size of greater than 10 nanometers, and wherein thephotoluminescent peak of the quantum dots is present in a wavelengthrange of greater than or equal to about 430 nanometers and less than orequal to about 470 nanometers.
 21. The quantum dots of claim 20, whereinthe core comprises ZnSe_(1-x)Te_(x), wherein, x is greater than or equalto 0 and less than or equal to about 0.05.
 22. The quantum dots of claim20, wherein in the semiconductor nanocrystal shell, an amount of thesulfur increases toward a surface of the quantum dots.
 23. The quantumdots of claim 20, wherein the semiconductor nanocrystal shell comprisesa first layer disposed directly on the core and an outer layer disposedon the first layer, and wherein the first layer comprises a secondsemiconductor nanocrystal having a composition different from acomposition of the first semiconductor nanocrystal and the outer layercomprises a third semiconductor nanocrystal having a compositiondifferent from the composition of the second semiconductor nanocrystal.24. The quantum dots of claim 23, wherein the first semiconductornanocrystal comprises ZnSe_(1-x)Te_(x), wherein, x is greater than 0 andless than or equal to about 0.05, the second semiconductor nanocrystalcomprises a ZnSe, and the third semiconductor nanocrystal comprises aZnS and does not comprises selenium.
 25. The quantum dots of claim 23,wherein an energy bandgap of the first semiconductor nanocrystal is lessthan an energy bandgap of the second semiconductor nanocrystal and theenergy bandgap of the second semiconductor nanocrystal is less than anenergy bandgap of the third semiconductor nanocrystal.
 26. The quantumdots of claim 23, wherein an energy bandgap of the second semiconductornanocrystal is less than an energy bandgap of the first semiconductornanocrystal and an energy bandgap of the third semiconductornanocrystal.
 27. The quantum dots of claim 20, wherein the quantum dotshave an average particle size of greater than or equal to about 15nanometers.
 28. The quantum dots of claim 20, wherein the quantum dotshave an average value of solidity of greater than or equal to about0.9:1.
 29. The quantum dots of claim 20, wherein the quantum dots have aquantum efficiency of greater than or equal to about 40% and a fullwidth at half maximum of less than or equal to about 30 nanometers. 30.A composition comprising the quantum dots of claim 20 and an organicsolvent.